1. Field of the Invention
The present invention relates to a method of applying a coating fluid such as photoresist liquid agent or polyimide resin onto the surface of a substrate such as a liquid crystal glass plate, an image sensor substrate, or a semiconductor substrate using a roll coater. More particularly, the present invention relates to a method of forming a thin film of uniform and predetermined thickness on the surface of a substrate with an applicator roll by bringing into contact or depressing and rotating the applicator roll relative to the surface of the substrate while the latter is conveyed in a horizontal direction.
2. Description of the Related Art
A conventional roll coater used for such an application is disclosed in Japanese Utility Model Laid-Open No. 2-133470, for example. This conventional roll coater includes a stage for holding a substrate by vacuum suction and transporting the same in a horizontal direction, a reservoir for storing coating fluid, and an applicator roll for applying onto the surface of a substrate coating fluid supplied from the reservoir through one or a plurality of coating fluid supplying rolls.
At least the cylindrical surface of the circumference of the applicator roll which holds a thin film of the supplied coating fluid is formed of an elastic material such as a rubber material.
Using a conventional roll coater, a thin film such as a photoresist film of a predetermined thickness is formed on the surface of a substrate as set forth in the hereinafter.
A substrate is held by vacuum suction on a stage and is conveyed in a horizontal direction by the stage. The applicator roll is rotated to have its circumferential cylindrical surface in contact with or depressed against the surface of the conveyed substrate, whereby the coating fluid is transferred from the circumferential cylindrical surface of the applicator roll to the surface of the substrate. When the circumferential cylindrical surface of the applicator roll is depressed against the surface of the substrate, a portion of the circumferential cylindrical surface thereof is deformed in an elastic manner.
With reference to FIG. 1 it is seen that by utilizing such a conventional method the coating fluid is applied in a greater amount at the peripheral edge of a substrate 1. Although undesirable, the rear or trailing of substrate 1 in the transportation direction x will have a greater amount of coating fluid applied to result in the formation of a projection 6. It is assumed that the thickness of projection 6 is t.sub.1, and the thickness of a thin film 5 formed at the center portion of substrate 1 is t. The thickness t.sub.1 is approximately 1.5 to 3 times the thickness of t.
Such a phenomenon was conventionally considered to be inherent in coating by using a roll coater. It was considered that this undesirable result or problem could not be avoided as long as a roll coater is used.
A substrate having a thin film formed thicker at the rear edge than at other portions will result in the following problem when a photoresist is employed as the coating fluid and following application of the coating fluid onto a substrate, contact exposure is carried out. In this process, sufficient adhesion between the exposure mask and the surface of the substrate cannot be obtained due to the above-described projection 6. As a result, the quality of the exposure is reduced. Furthermore, in the developing process following the degraded contact exposure degraded, there is a possibility that the projection 6 of the photoresist film on the substrate surface is not sufficiently developed and remains on the surface of the substrate 1.